During the past two decades the science and implementation of in situ groundwater remediation through the subsurface injection of liquid remedial agents has advanced substantially, and these techniques are now successfully employed in many settings. Most commonly, these strategies involve the injection of a specialized reagent solution into an aquifer in order to create a zone within the aquifer where enhanced biotic and/or abiotic reactions occur in situ to detoxify or destroy target contaminants. This general strategy is often referred to as an in situ reactive zone (IRZ) approach.
Enhanced reductive dechlorination (ERD) is one of the most widely-applied methods for treating chlorinated volatile organic compound (CVOCs) contaminants. In practice, ERD typically involves injecting an electron donor (usually a biodegradable form of organic carbon such as molasses, corn syrup, or vegetable oil) into the subsurface. Other similar commonly-employed strategies include the use of electron donors to support the biologically-mediated in situ conversion and precipitation of hexavalent chromium to trivalent chromium, and the injection of various chemical oxidants for the direct oxidation of applicable contaminants.
Although several important factors affect the implementability and potential success of an injected remedial fluid-based approach, relatively high hydraulic conductivity site conditions is one of the most important and desired site features, because this property controls the ease at which remedial fluids can be injected. For example, literature describing in situ remediation system design considerations identifies high aquifer hydraulic conductivity as a critical site property and screening criterion. See, e.g., Morse, J. J., Alleman, B. C., Gossett, J. M., Zinder, S. H., Fennelll, D. E., Sewell, G W., and Vogel, C. M., 1998. Draft technical protocol: A Treatability test for evaluating the potential applicability of the reductive anaerobic biological in situ treatment technology (RABITT) to remediate chloroethenes. Prepared for Environmental Security Technology Certification Program (ESTCP); Suthersan, S. S., 2002. Natural and enhanced remediation systems. CRC Press, Lewis Publishers, Boca Raton, Fla., 419 pp.; Gossett, J. M., Zinder, S. H., Fennel, D. E., Morey, C., and Adamson, D. T., 2003. Cost and performance report: Reductive anaerobic biological in situ treatment technology (RABITT) treatability testing. Prepared for Environmental Security Technology Certification Program (ESTCP); Faris, B., Vlassopoulos, D., and ITRC—In Situ Bioremediation Team, 2003. A Systematic approach to in situ bioremediation in groundwater. Remediation Journal 13: 27-52; Payne, F. C., and S. S. Suthersan., 2005. In Situ Remediation Engineering. CRC Press, Boca Raton, Fla. 511 pp; Payne, F. C., J. Quinnan, S. T. Potter. 2008. Remediation Hydraulics. CRC Press, Boca Raton, Fla.
In most cases, sites with low natural bulk hydraulic conductivity values (typically less than approximately 5 ft/day) are not considered ideal candidate sites for injected liquid-reagent-based in situ remedial strategies. This is primarily because at these sites, injection times necessary to deliver adequate reagent volume under non-fracturing pressures are prohibitively long. Consequently, many sites which might otherwise be good candidates for these types of strategies (that is, the contaminants are appropriate, geochemical conditions are favorable, and site use/infrastructure is compatible) are eliminated from conventional IRZ approaches or deemed less suitable solely because of the hydraulic challenges associated with remedial amendment delivery and distribution.
Injected reagent-based strategies are also challenged by the difficulty in delivering and distributing the reagent to the target zones within the plume. To some degree, this challenge has been overcome by optimizing the delivery and distribution of soluble substrates, through a renewed focus on the fundamental properties that govern groundwater movement and solute distribution at the remedial system scale. For example, it is now understood that large injection volumes are necessary to achieve lateral coverage due to the recognition that transverse dispersion of injected reagent is generally insignificant. See, e.g., Payne et al., 2008, supra. However, an important implication of this concept is that conventional batch-volume injection methods will not be technically-appropriate or cost-effective for large plumes. Large plume treatment with injected fluid reagents can only be practically accomplished when the flow field is artificially controlled by extraction-injection systems that facilitate larger well spacing. See, e.g., Suthersan, S. S., C. E. Divine, and S. T. Potter. 2009. Remediating large plumes: Overcoming the scale challenge. Ground Water Monitoring & Remediation 29, no 1: 34-43.
Furthermore, it is increasingly recognized that contaminant mass flux and discharge information provides the most useful measure of plume strength and potential risk to off-site receptors, and remedial approaches that focus on flux will increasingly be preferred to most effectively reduced risk. See, e.g., Nichols, E. M., 2004. In a state of (mass) flux. Ground Water Monitoring & Remediation 24, no. 3: 4-5; Suthersan, S. S., C. Divine, J. Quinnan, and E. Nichols. 2010. Flux-informed remediation decision making Ground Water Monitoring & Remediation 30, no 1: 45-50.
Finally, despite the widespread success of injected remedial reagent approaches for select contaminants, there still remain a large number of contaminants (e.g., select metals and radionuclides, polycyclic aromatic hydrocarbons, explosives and energetics, chlorides) that are not treatable, or their treatment is relatively undemonstrated by currently-available liquid-based remedial agents.
Additionally, while many soluble substrates (i.e., lactate, molasses and other organic carbon sources) are ideally suited to an injection approach, they are not ideal with respect to reaction rates since their effectiveness in contaminant destruction relies on promoting microbiological activity.
The concept of placing reactive media in vertically-oriented passive wells has been previously theoretically evaluated. See, e.g., Wilson, R. D., D. M., Mackay, and J. A. Cherry. 1997. Arrays of unpumped wells for plume migration control by semi-passive in situ remediation. Ground Water Monitoring & Remediation 21, no. 3: 185-193; Hudak, P. F. 2008. Evaluation of reactive well networks for remediating heterogeneous aquifers. J Environ Sci Health 43, no 7: 731-737; Hudak, P. F. 2009. Interior versus exterior configurations of passive wells with filter cartridges for cleaning contaminated groundwater. See Remediation 20, no. 1: 133-141. While theoretically plausible, the concept is not practical or cost-effective for most sites because the treatment widths for individual reactive wells are very small (roughly twice the well diameter) due to negligible flow-focusing, and therefore, many wells would be necessary to achieve a system width appropriate for typical plumes. Furthermore, the in-well residence times for these systems are very short (a few hours to a few days) and this may not be long enough to achieve treatment of some contaminants.
Passive reactive barrier (PRB) technology is well-developed and numerous field applications have been installed over the past decade. See USEPA 1999. Field applications of in situ remediation technologies: Permeable reactive barriers. EPA542-R-99-002, April 1999. Generally, PRB design consists of a reactive media-filled trench oriented orthogonal to the groundwater flow direction and is intended primarily to “cut off” the plume and control contaminant mass discharge at a discrete transect location, and many implementations utilize a “funnel-and-gate” system configuration to focus the treatment zone and reduce the volume of reactive media needed. Solids that have been used for in situ remediation include iron particles, limestone rock, oxide minerals, and particulate organics (e.g., mulch). These are generally emplaced in a trench-type PRB across the path of groundwater with the reactive solid media mixed with sand or gravel. However, this approach at applying solids in the treatment of plumes is not ideal: trenching is an expensive and difficult operation, reactive solid media have limited life-span and rehabilitation of the barrier is often not feasible, and often large portions of a plume cannot be treated because of limitations on trenching at a site. Other reactive media are also not available for use in trenches, such as ion-exchange resins and granular activated carbon (solids used successfully in ex-situ applications) due to the media cost and the need for large quantities to fill a trench.
Some limited work has been performed to evaluate the hydraulic performance of trench-based PRB systems oriented at non-orthogonal angles. See Edwards, D. A., and V. V. Dick. 1998. Method for directing groundwater flow and treating groundwater in situ. U.S. Pat. No. 5,833,388. Issued November 10; Edwards, D. A., V. V. Dick, J. W. Little, and S. L. Boyle. 2001. Refractive flow and treatment systems: Conceptual, analytical, and numerical modeling. Ground Water Monitoring & Remediation 21, no. 3: 64-70. Edwards et al. present a limited hydraulic capture analysis of a technique termed “refractive flow and treatment” (RFT), which consisted of fully-penetrating trenches installed in bedrock at oblique angles to ambient groundwater flow direction, and filled with highly-permeable inert material. The authors simulated the flowfield and hydraulic capture associated with “X” and chevron-shaped trench orientations and demonstrated that the trenches induce flow-focusing behavior. In concept, this captured water could be routed to a discrete zone where groundwater would be treated in situ by several possible technologies. Very recently, Hudak conducted some limited modeling of hypothetical reactive media-filled fully-penetrating PRBs used to treat the leading edge of synthetic plumes. See Hudak, P. F., 2010. Viability of longitudinal trenches for capturing contaminated groundwater. Bull Environ Contam Toxicol. Therein, Hudak evaluated several alternative orientations and his results suggest that PRB trenches oriented parallel to groundwater flow could result in faster treatment for the entire synthetic plume due to shorter average travel distances to the trench and associated reactive material.
While further work is needed to evaluate the potential benefits and the limitations of alternative orientations, there are important cost and implementability limitations associated with conventional PRBs. Trenching is relatively expensive as costs are particularly sensitive to trench depth. Additionally, the practical maximum trench depth attainable by available trenching technology is about 100 ft. Further, there are challenges associated with achieving adequate contaminant residence times for conventional orientations, and the hydraulic performance of many existing systems has been impacted by clogging and reduction in permeability, which can result in undesirable alteration of the flowfield and even plume spreading and bypass around the PRB. Reduction of hydraulic conductivity and reactivity in zero-valent iron columns by oxygen and TNT. Ground Water Mon. & Remd. 25(1): 129-136. Lastly, access issues such as buildings and utilities may limit the use of trenches. See, e.g., Johnson, R. L., P. G. Tratnyek, R. Miehr, R. B. Thoms, and J. Z. Bandstra, 2005.